Infrared cameras operating in the thermal infrared portion of the electromagnetic spectrum present images of scenes in which the image is formed by the optical system mounted in front of the camera, the foreoptics. Such infrared cameras incorporate cold detectors that are sensitive to thermal radiation. As is known all objects emit infrared radiation in proportion to their temperature in accordance with the Planck blackbody function. Thus parts of the camera or the foreoptics can be a source of unwanted infrared radiation if protective measures are not undertaken. To prevent the cold detector in the infrared camera from receiving unwanted radiation from the camera structure, the detector is typically surrounded inside a structure, termed the “radiation shield,” that is at a very cold in temperature. In turn, the radiation shield is enclosed inside a vacuum chamber. The vacuum chamber contains a window, impervious to ambient air, which transmits exterior infrared radiation received at the window. The infrared radiation from the scene collected by the optics passes through the vacuum window, and through an aperture in the radiation shield, and is then focused at the cold detector.
The aperture in the radiation shield is located at the exit pupil of the front optical system or foreoptics. That aperture is sized so that it admits infrared radiation propagating from the foreoptics only, and blocks radiation from the surrounding structure, including the mechanical structure of the camera supporting the optics, and the vacuum chamber walls. This aperture is referred to as a stop, and more specifically, as a cold stop, because (i) the aperture admits the desired infrared radiation, but stops all undesired radiation, and (ii) the aperture is cold so that the structure around the stop does not contribute to the undesired radiation.
Normally, the f-number of the optics is matched to the cold stop f-number. If there is a change in the optics that results in a different field of view, then the f-stop number of the optics is changed and cold stop f-number must also be changed to match. In modern infrared systems, the optics of the camera is capable of changing the observed field of view. As a result, when the field of view is changed, the f-number of the optics may change. When the f-number of the optics changes, the aperture in the cold stop no longer matches the exit pupil size of the optics. The cold stop aperture may then be too large or too small. If it is too small, the aperture blocks some of the desired radiation from entering the detector, causing vignetting. If the aperture is too large, it admits too much of the unwanted radiation, which degrades the image.
A solution to such problems previously invented by some inventors of the present invention has been to use a continuous variable size aperture in the radiation shield and to effectively match the size of that aperture to the f-number of the front optics. In this discussion the terms aperture stop and exit pupil refer to the same physical position in the system optical train. In prior work, circular variable aperture cold stops were developed and matched to the virtual aperture size or f-stop defined by the front optics or lens as that which obtains the clearest image and that is the position where the rays of light that pass through the lens effectively collect and define an aperture (see U.S. Pat. No. 7,157,706 to N. Gat (the “'706 patent”), and Ser. No. 11/273,919, filed Nov. 14, 2005, currently published U.S. Application no. 20060255275 to Garman et al. both of which are assigned to the assignee of the present invention).
In some modern infrared systems, the exit pupil of the front optics is found to be of a non-circular area or geometry, such as the shape of a rectangle, a square, or a racetrack (a rectangle with rounded corners), because of limited optical aperture size that can be supported by the vehicle on which the FLIR system is mounted. The existence of those non-circular exit pupils evidences a need for a corresponding non-circular variable aperture cold stops if the infrared camera is to permit matching the size of the aperture in the radiation shield with the f-number of the front optics as in the prior invention in the '706 patent in those infra red cameras with foreoptics that form a non-circular exit pupil.
The variable aperture or iris of a camera is typically created by the use of multiple thin blades that are pivoted at one end, while the other end is rotated by an actuator. The actuator contains slots in which a pin that is attached to the blade, the actuator pin, is received and dragged about by the actuator. The actuator motion causes the blades to pivot and form the aperture into a larger or smaller size circle, depending on the direction of rotation of the actuator. The diameter of that circular aperture depends on the degree of actuator rotation with the largest aperture formed at the maximum rotational position in one direction and the smallest aperture formed at the maximum rotational position in the opposite direction. Such circular iris mechanisms are well known in the photography world. Those circular iris mechanisms are also seen to be continuous. That is, the geometry or shape of the aperture remains unchanged even though the aperture increases (or decreases) in size with the degree of rotation of the actuator. Some photographic lenses contain only a few blades (say 5 or 6, as example) in the iris mechanism. As a result the shape of the aperture may be an imperfect circle, such as a pentagon or a hexagon or other convex polygon with equal length sides. With greater number of blades in the iris, and with slightly curves edges to the blades, the circular symmetry of the aperture improves, forming heptagon, octagon, etc., with more rounded corners. Such mechanisms, however, are not amenable to produce continuous variable rectangular, square or racetrack shaped apertures.
Circular or nearly circular variable apertures are well known in prior art. Apertures that produce discrete non-circular openings in infra red camera systems have also been produced in the past. For example in the fully open position, the blades that form the aperture retract out of the way, revealing an underlying (or overlying) square or rectangular opening defined by an opening in the radiation shield or some other fixed structure. As the blades are rotated inwardly, that is, rotated clockwise or counter-clockwise in dependence on the particular mechanical design chosen for the mechanism, the blades produce a smaller size non-circular aperture. The problem with the foregoing apertures that produce non-circular openings is that they provide limited discrete aperture sizes, typically limited to two aperture sizes in the requisite geometry or shape, but are not continuous. Were the foregoing blades permitted to also remain open at locations in between the two positions of full open aperture or fully closed-down aperture, one would find that the geometry or shape of the aperture at those in-between positions are no longer the same as the shape at the full open and full closed position, but is different, and changes even further as one continues to advance the rotation of the actuator. Because the shapes assumed by the aperture in the in-between positions varies widely and differs from the desired shape that matches the aperture shape defined by the optics, the foregoing designs for the non-circular apertures are unacceptable. Such changes of aperture shape in the foregoing context are regarded as discontinuous. The object of the present invention is to provide a continuous variable aperture that may be adjusted to any desired size of a defined aperture shape between and including the largest and smallest opening desired in the optics of the camera. Moreover, in a special case the smallest size of the aperture may be set to completely block the incoming radiation, serving as a cold shutter for the camera to permit use in camera calibration and also for blocking undesired intense radiation that could potentially blind or even damage the infra-red sensor of the camera.